Thermocouples using noble metals are commonly employed in high temperature measurement and control, particularly for temperatures of 2000.degree. F and higher. Typically, such thermocouples use platinum as the negative thermoelement and an alloy of platinum and rhodium as the positive thermoelement. Two different alloy compositions are normally used; the first contains 13% rhodium, balance platinum, and the second contains 10% rhodium, balance platinum. A thermocouple using platinum as the negative thermoelement together with a positive thermoelement having the first composition is known as a Type R thermocouple. When the second composition is substituted for the first composition, the resultant thermocouple is known as a Type S thermocouple.
Thermocouples are usually spaced from the measurement or control instrumentation by distances which can be six feet or more, and thus thermocouple extension wires (or extension wires) are used to connect the thermocouples to the instrumentation. Because of the high cost of platinum, these extension wires to the platinum thermocouples are composed of less expensive metals or alloys.
It is well known to use as extension wires, for both Type R and Type S thermocouples, a copper wire as the positive thermo-couple extension (or lead) wire and a copper-nickel alloy wire, known as PCLW (platinum compensating lead wire) as the negative thermocouple extension wire. This last named alloy contains about 0.75% nickel, balance copper.
In the normal installation, the thermocouples are connected to a header junction, and the extension wires are connected between the header junction and the instrumentation. In most industrial applications, the temperature of the header junction will not exceed 400.degree. F while the temperature at the instrumentation will not fall below 32.degree. F.
Ideally, to avoid inaccuracy, the extension wires and a noble thermocouple should have matched thermoelectric characteristics at which the differential electromotive force (EMF) developed between the two extension wires should be equal, both in polarity and magnitude, to the differential EMF developed between the two thermocouple wires at any temperature within the range 32.degree. F to 400.degree. F.
I have found that the use of copper-nickel alloy wire as a negative thermocouple extension wire together with a copper wire as the positive thermocouple extension wire does not provide an ideal match.
In my invention, I employ a new alloy composition for the negative extension wire which, when used together with a copper wire as the positive extension, produces a much better match not only for Type S thermocouples but for Type R thermocouples as well.